THE SENSES 



153 



FlG. 125. Change of lens in accom- 

 modation. (Jegi.) 



Accommodation. - - In order to focus the light upon the 

 retina, the lens must change shape for every change in the 

 distance of the object looked 

 tf/(see Fig. 125). The shape 

 of the lens can be readily 

 changed, for it is elastic and 

 has muscular fibers around 

 its edges (Exp. 17). 



Defects in the Eye.- -Some 

 eyeballs are too long, and the lens brings the rays to a 

 focus before they reach the retina. Such eyes are near- 

 sighted (Fig. 126) and require 

 glasses that round inward (con- 

 cave). Some eyeballs are too 

 flat, and the rays are not brought 

 to a focus soon enough. Such 

 eyes are farsighted and require 

 glasses that round outward 

 (convex). See Fig. 127. (Re- 

 peat Exp. 15.) 



Care of the Eyes. - - Because 

 the eyes can do a large amount 

 of work without giving pain, 

 they are often abused. When 

 reading or doing intricate work, turn the eyes from the 



work occasionally and look 

 at some distant object ; stop 

 work before the eyes are 

 tired. Twilight of early 

 evening has ruined many 



good eyes. You should 

 FIG. 127. --FARSIGHTED EYE (ball a] g st work before 



too short) which needs convex lens 



to focus rays upon retina. the twilight begins, for the 



FIG. 126. -- (i) NEARSIGHTED 

 EYE (ball too long), which only 

 focuses rays for near objects 

 (2) when concave glasses are 

 used (3). 



